1. Field of the Invention
The present invention relates to a steering head structure for steerably connecting a front wheel to the body frame of a bicycle.
2. Description of the Related Art
As shown in FIG. 7, a steering head structure designed not only for a bicycle but for a motorcycle or a motor scooter has a lower bearing device 6 and an upper bearing device 10. The lower bearing device 6 includes a lower inner race 2 fitted on a fork stem 1, a lower outer race 4 fitted in a lower end of a head pipe 3, and balls 5. The upper bearing device 10 includes an upper inner race 7 fitted in an upper end of the head pipe 3, an upper outer race 8 fitted on the fork stem 1, and balls 9.
In the conventional steering head structure as noted above, a clamping nut and locknut are screwed to an upper position of the fork stem projecting above the upper outer race. A pressure of contact between the balls and races of the upper and lower bearing devices is determined by a clamping force of the clamping nut. If the clamping force is insufficient, steering control becomes unstable with chattering of the bearing devices, and collisions between balls and races may damage sliding surfaces thereof. If the clamping force is too strong, the steering control becomes heavy, and the balls could inflict indentations on the sliding surfaces of the races to impair a smooth steering control. It is therefore important to fix the clamping nut with a proper tightening torque.
In an actual assembly operation, the nut must be tightened while checking if a light steering control may be made without chattering, i.e. taking the ball contact pressure into account. Thus, the operation to assemble the steering head structure is relatively difficult and irksome.